This paper presents the second data release of the Hyper Suprime-Cam Subaru Strategic Program, a wide-field optical imaging survey using the 8.2 m Subaru Telescope. The release includes data from 174 nights of observation through 2018 January. The Wide layer data cover about 300 deg$^2$ in all five broad-band filters ($grizy$) to the nominal survey exposure (10 min in $gr$ and 20 min in $izy$). Partially observed areas are also included in the release; about 1100 deg$^2$ is observed in at least one filter and one exposure. The median seeing in the i-band is ${0_{.}^{\prime \prime }6}$, demonstrating the superb image quality of the survey. The Deep (26 deg$^2$) and UltraDeep (4 deg$^2$) data are jointly processed and the UltraDeep-COSMOS field reaches an unprecedented depth of $i\sim 28$ at $5 \, \sigma$ for point sources. In addition to the broad-band data, narrow-band data are also available in the Deep and UltraDeep fields. This release includes a major update to the processing pipeline, including improved sky subtraction, PSF modeling, object detection, and artifact rejection. The overall data quality has been improved, but this release is not without problems; there is a persistent deblender problem as well as new issues with masks around bright stars. The user is encouraged to review the issue list before utilizing the data for scientific explorations. All the image products as well as catalog products are available for download. The catalogs are also loaded into a database, which provides an easy interface for users to retrieve data for objects of interest. In addition to these main data products, detailed galaxy shape measurements withheld from Public Data Release 1 (PDR1) are now available to the community. The shape catalog is drawn from the S16A internal release, which has a larger area than PDR1 (160 deg$^2$). All products are available at the data release site, https://hsc-release.mtk.nao.ac.jp/.

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Gold nanoparticles used in many types of nanostructure are mostly stabilized by citrate ligands. Fully understanding their dynamic surface chemistry is thus essential for applications, particularly since aging is frequently a problem. Using surface-enhanced Raman spectroscopy in conjunction with density functional theory calculations, we are able to determine Au–citrate coordination in liquid with minimal invasiveness. We show that citrate coordination is mostly bidentate and simply controlled by its protonation state. More complex binding motifs are caused by interfering chloride ions and gold adatoms. With increasing age of stored gold nanoparticle suspensions, gold adatoms are found to move atop the Au facets and bind to an additional terminal carboxylate of the citrate. Aged nanoparticles are fully refreshed by removing these adatoms, using etching and subsequent boiling of the gold nanoparticles.